1. Field of the Invention
The present invention relates to frequency modulated oscillators and more particularly to a voltage-controlled oscillator that is capable of very high ratios of modulation to oscillator frequency.
2. Prior Art
In frequency modulation (FM), a source of oscillating electrical waves is used to produce an electrical wave at a selected frequency of oscillation (carrier wave) and a modulating signal is imposed on the carrier wave to alter the selected frequency. Typically, the carrier wave is transmitted at a frequency within the radio frequency (RF) band and is picked up by a receiver which removes and uses the modulating signal to reproduce the source of the modulation, such as voice or music sounds or a video image. In its simplest form, the source of the oscillating waves, the oscillator, is composed of a parallel connected electrical inductance and capacitance, the operating values of which are selected to produce the desired frequency of oscillation. Essentially, the parallel connected inductance and capacitance, which are commonly referred to as a tank circuit, trap the energy of an applied voltage and produce a resonating effect that results in the oscillating voltage output at the resonant frequency. Such tank circuits have a quality or Q rating associated with them proportioned to their ability to store electrical energy and thus, produce a strong stable output signal.
A particular prior art oscillator, varactor tuned with a resistive bias network, is shown in FIG. 1. The tank or resonant circuit T.sub.A is formed by the inductance coil L.sub.A and the capacitors C.sub.A and D.sub.A. Capacitor D.sub.A is a voltage-variable capacitor or varactor diode which will effect the resonant frequency of the circuit as a function of the applied voltage. A feedback loop consisting of an inductance L.sub.F and an amplifier A.sub.F, with an inverted gain -A, is connected across inductance L.sub.A. A source of tuning voltage V.sub.T is coupled to the oscillator circuit at a point between capacitors C.sub.A and D.sub.A and normally a resistor network R.sub.A is used to isolate the source of tuning voltage V.sub.T from the hi-Q resonant tank circuit of the oscillator. This prior art arrangement is necessarily a compromise because:
1) The presence of the resistor network R.sub.A causes the Q of the resonant tank circuit to be lowered, which compromises the stability and purity of the oscillator's output waveform; and
2) The impedance of the resistor network together with circuit capacitance comprise a low-pass filter that ultimately limits the response of the oscillator circuit to high frequency modulation inputs.
It is accordingly an object of the present invention to provide a voltage-controlled oscillator with an increased modulation bandwidth that overcomes the limitations of the prior art.